1. Field of the Invention
This invention relates to an optical waveguide of fused silica fiber having a cylindrical layer of high refractive index for use in an optical transmission system.
The optical waveguide of the present invention consists of a fused silica fiber of an extremely high light transparent material having a cylindrical layer of the same material, but higher in refractive index by several % at the most than that of the fused silica fiber interposed between the center and outer circumference of the fiber.
In such an optical waveguide, it is known that the dominant energy of the transmitted light is present in the thin cylindrical layer of higher refractive index.
The theoretical analysis of such an optical waveguide is reported by the technical paper No. 852 "Transmission Properties of the optical fiber having thin cylindrical layer of a higher refractive index than that of the fiber material" of the General Conference of 1972 and the technical paper No. 995 "The Transmission properties of HE.sub.11 mode in the optical fiber which has a cylindrical thin layer of a higher refractive index than that of the fiber material" of the General Conference of 1973, of the Institute of Electronics and Communication Engineers of Japan.
2. Description of the Prior Art
For the manufacture of an optical waveguide of a fused silica fiber having a thin cylindrical layer of higher refractive index in it, a method has been known in which either a transparent fused silica rod is inserted inside a transparent fused silica tube which has a cylindrical uniform layer of high refractive index attached to its inner surface, or a transparent rod which has a cylindrical uniform layer of high refractive index attached to its outer surface is inserted coaxially inside a transparent fused silica tube and the end portions of them are heated to the melting point thereof and drawn to a fiber with a constant drawing speed.
With an optical waveguide manufactured in this way, however, there takes place an inevitable scattering loss of light at the inner or outer surfaces of the cylindrical thin layer of higher refractive index owing to the roughness of the interfaces of the cylindrical thin layer or owing to the inclusion of air and foreign matter which occasionally comes to exist between the thin layer and the rod or tube during manufacture.